Hielke A. Jelsma

Horizontal accretion and stabilization of the Archean Zimbabwe Craton

Structural-metamorphic data and mineral ages from the northern parts of the Zimbabwe craton indicate that Archean crustal formation and stabilization evolved in two stages. In the Shamva-Bindura greenstone belt, early layer-parallel shear zones began to form at 2670 Ma and accommodated imbricate stacking of oceanic and volcanic arc material between large nappe structures with felsic gneiss cores. The resultant crustal pile of anomalously hot felsic and mafic crustal slices reached isostatic and mechanical equilibrium at a thickness of 35 km. Further shortening of this pile caused strain partitioning into vertical strike-slip zones. The subsequent establishment of an equilibrium geotherm resulted in large-scale crustal melting and diapirism between 2620 and 2600 Ma. The rise of the melt and diapirs caused a second transient metamorphic imprint and much of the strain pattern regarded as typical for the Zimbabwe craton. This diapiric stage led to cooling and stabilization of the craton.

U–Pb and 40Ar/39Ar geochronological constraints on the tectonic evolution of the easternmost part of the Zambezi orogenic belt, northeast Zimbabwe

Abstract The Zambezi belt is a key segment of the network of Neoproterozoic/lower Paleozoic orogenic belts in southern Africa that formed during amalgamation of central Gondwana. We present new geochronological data from the easternmost Zambezi belt in northeast Zimbabwe, near the junction between that belt and the Mozambique belt. Allochthonous high-pressure granulite-facies migmatitic and mylonitic rocks at the highest exposed structural levels in this part of the Zambezi belt are tectonically juxtaposed against amphibolite-facies supracrustal rocks, which are intruded by a regionally extensive, sheet-like composite granitoid batholith having peralkaline affinities. This batholith separates the Zambezi supracrustal rocks from a zone of Archean basement that wraps the northeast margin of the Zimbabwe craton and shows variable degrees of structural and thermal overprinting. U–Pb zircon and titanite and 40 Ar/ 39 Ar hornblende geochronological data from the allochthonous granulites are interpreted to record high-grade migmatization at ca 870 to 850xa0Ma, with pervasive amphibolite-facies retrogression at ca 535xa0Ma during tectonic emplacement into mid-crustal levels. U–Pb zircon and titanite data from the peralkaline batholith indicate that it crystallized at 805.2±11.1xa0Ma; partial thermal resetting of the U–Pb system occurred in the same time frame as retrogression of the allochthonous granulites. Hornblende from two samples in thermally overprinted Archean basement farther south yields 40 Ar/ 39 Ar plateau ages of 507.9±2.5 and 491.3±2.1xa0Ma. Together these new data indicate that tectonic elements in the easternmost Zambezi belt have a protracted history, involving early, lower crustal, granulite-facies metamorphism (ca 870–850xa0Ma) followed by intrusion of a peralkaline granitic batholith (ca 800xa0Ma) into supracrustal rocks within the belt. The major tectonostratigraphic units in the easternmost Zambezi belt were juxtaposed under amphibolite-facies conditions at ca 535xa0Ma, followed by relatively rapid cooling through Ar closure temperatures in hornblende. The 535xa0Ma event reflects deformation in the Zambezi belt in the same time frame as widespread orogenesis that is recorded in other Pan-African belts in southern Africa and is related to final stages in Gondwana assembly.

Late Archaean foreland basin deposits, Belingwe greenstone belt, Zimbabwe

Abstract The c. 2.65xa0Ga old sedimentary Cheshire Formation of the Belingwe greenstone belt (BDB), central Zimbabwe, has been studied in detail for the first time to shed some light on the much debated evolution of this classical belt. The Cheshire Formation rests sharply on a mafic volcanic unit (Zeederbergs Formation) and comprises a basal, eastward-sloping carbonate ramp sequence built of shallowing-upward, metre-scale sedimentary cycles. The cycles strongly resemble Proterozoic and Phanerozoic carbonate cycles and might have formed by small-scale eustatic sea level changes. The top of the carbonate ramp is represented by a karst surface. The carbonates are overlain by and grade laterally to the east into deeper water (sub-wave base) siliciclastic facies. Conglomerate, shale and minor sandstone were deposited by high- to low-density turbidity currents and were derived from the erosion of Zeederbergs-like volcanic rocks from the east. Shortly after deposition, the Cheshire Formation and underlying volcanics were affected by a northwest-directed thrusting event. Thrusting gave rise to the deformation of semi-consolidated sediments and resulted in the juxtaposition of a thrust slice of Zeederbergs basalts onto Cheshire sediments. The stratigraphy, asymmetric facies and sediment thickness distribution, palaeogeographic constraints and evidence for an early horizontal tectonic event suggest that the Cheshire Formation formed in a foreland-type sedimentary basin.

Structural relations and PbPb zircon ages for the Makuti gneisses: evidence for a crustal-scale Pan-African shear zone in the Zambezi Belt, northwest Zimbabwe

Abstract The Makuti Group of northwest Zimbabwe is composed of mafic and intermediate biotite-rich gneisses interlayered with quartzofeldspathic gneisses of granitic composition, and minor sedimentary units. The gneisses have experienced a multi-staged metamorphic history, including an early high temperature-high pressure event and subsequent reworking at upper- to mid-amphibolite-facies conditions. They are positioned along the strongly deformed, southern margin of the east-west trending Zambezi Belt, and have been correlated with supracrustal gneiss units along the northern margin of the Zimbabwe Craton. The Makuti Group is characterised by an intensely developed gneissic layering and complex disharmonic folds that resulted from non-coaxial deformation involving repeated stages of transposition. The basal contact of the g roup coincides with a decrease in strain intensity, but not with a directional change of characteristic structural elements (e.g. lineations, fold axes), nor with a clear change in rock types. Pink quartzofeldspathic gneisses of granitic composition are typical for the Makuti Group, but locally intrude basement gneiss as well. The quartzofeldspathic gneisses occur as porphyritic and non-porphyritic varieties that are, invariably, intensely sheared. The age and nature of the basal contact of the Makuti Group and its relationship to the quartzofeldspathic gneisses has been investigated. Samples for single zircon Pbue5f8Pb dating were collected from a felsic biotite gneiss just below (2704 ± 0.3 Ma) and above (2510 ± 0.4 Ma) the lower contact of the Makuti Group at an ‘unconformity’ 2 km northwest of Vuti. Further samples were collected from pink quartzofeldspathic units at the base (737 ± 0.9 Ma), central part (764 ± 0.9 Ma; 797 ± 0.9 Ma) and top (794 ± 0.5 Ma; 854 ± 0.8 Ma) of the Makuti Group. Two samples of Kariba orthogneiss (1920 ± 0.4 and 1963 ± 0.4 Ma) underlying the Makuti Group in the northwest were also collected. In all samples, long-prismatic, colourless to brown, igneous zircon grains were selected. Dates were obtained using a stepwise single-grain evaporation technique. Although this technique only allows minimum age estimates, the dates are highly reproducible, indicating that they approximate emplacement ages. The ages conform with the field observations that the basement has been reworked in the Makuti Group and that the quartzofeldspathic units may have been emplaced as granites. It is proposed that the Makuti Group represents a crustal scale shear zone that partly reworked basement gneisses and acted as a conduit for granite emplacement. Shearing took place in an extensional setting around 800 Ma ago, and may have resulted in the exhumation of lower crustal rocks.

Silicic Layer-Parallel Shear Zones in a Zimbabwean Greenstone Sequence: Horizontal Accretion Preceding Doming

Abstract Detailed structural metamorphic data from the Trojan nickel mine area in the Shamva-Bindura Greenstone Belt (SBGB) show that two groups of ductile structures associated with separate amphibolite facies metamorphic assemblages can be distinguished. D 1 /M 1 structures are related to a network of anastomosing shear zones that accommodated W-directed imbricate stacking of the stratigraphy, while metamorphic conditions in the mine area reached about 500°C at pressures of 3-4 kbar. D 1 shears are locally strongly silicified and preserve fine-grained mylonitic textures. They have been wrongly identified in the past as banded iron stones or stratigraphic chert horizons. Truncations of primary layering associated with such fine-grained mylonitic quartzites are tectonic and not stratigraphic in origin, and the units can not be used as tectonic marker horizons, but instead represent glide planes across which tectonic imbrication of the stratigraphy has occurred. An important silicified shear zone of this nature occurs along the boundary of the Iron Mask and Arcturus Formations. D 2 /M 2 structures are related to doming of the Chinamora Batholith, and a contact metamorphic overprint and recrystallisation of M 1 assemblages. M 2 temperatures in the mine area reached 565°C. Such high contact metamorphic temperatures at 3 km from the contact of the batholith can only be explained if the entire Chinamore Batholith was emplaced as a relatively hot (>750°C) intrusive body in an already anomalously hot greenstone sequence. Metamorphic fluids during M 1 and M 2 where CO 2 -rich and carbonate alteration was pervasive. D 1 /M 1 and D 2 /M 2 events may have been separated by as much as 70 Ma. D 1 structures in the Trojan area can be related to a large mantled-gneiss, nappe structure represented by the Madziwa Batholith and a mantle of mafic greenstones to the N of the SBGB. The footwall thrust of this nappe occurs along the N boundary of the Shamvaian sediments in the centre of the SBGB. The imbricate stacking of the stratigraphy in the Trojan area can be interpreted as secondary structures in the footwall of the nappe. Horizontal accretion and stacking of crustal fragments during D 1 was followed by thermal perturbations that resulted in the diapiric rise of domes like the Chinamora Batholith during D 2 . After doming the terrain appears to have cooled and stabilized, with further deformation partitioned into narrow strike-slip shear zones.

Fluids and epigenetic gold mineralisation at Shamva Mine, Zimbabwe: a combined structural and fluid inclusion study

Abstract The Shamva gold mine is hosted within the Shamva greenstone belt and is related to steeply dipping reverse-oblique crustal shear zones. Fluid infiltration resulted in widespread sulphidisation, K alteration, chloritisation, silicification and carbonatisation. Fluid inclusions from two types of quartz veins were used to estimate the composition and pressure-temperature conditions of gold mineralisation: (1) Au mineralised sulphide ± carbonate bearing veins; (2) late tensional barren veins. The vein types contain aqueous, mixed H2Oue5f8CO2 (±CH4) and CO2 (±CH4) rich inclusions. Fluid inclusions in the mineralised and barren veins are similar in composition and resulted from trapping of an immiscible aqueous and CO2 fluid. The pressure-temperature conditions of Au mineralisation are constrained to 250–450°C and 1–3 kbar. The similar pressure conditions found for Au mineralisation and the peak of metamorphism implies that the retrograde pressure-temperature path followed isobaric cooling. Thermodynamic modelling of the fluid shows that: (1) XCO 2 (XCO 2 + XCH 4 ) of the metamorphic fluid ranged between 0.75 and 0.85; (2) XH2O of the Au mineralising fluid ranged between 0.85 and 0.90; and (3) metamorphic temperatures ranged between 500 and 530°C, assuming an oxygen fugacity buffered by quartz-fayalite-magnetite.

U-Pb zircon ages from a craton-margin archaean orogenic belt in northern Zimbabwe

Abstract In northern Zimbabwe, the Archaean Pfunzi Orogen comprises an east-west-trending belt of migmatitic gneisses separating the Zimbabwe Craton from the Pan-African Zambezi Belt farther north. Previously available Rb-Sr dates for the Pfunzi Orogenic Belt have been interpreted to record formation of granulites at ca 3.0 Ga and subsequent amphibolite-facies metamorphism at ca 2.6 Ga. Here the first conventional IDTIMS U-Pb zircon dates for the Pfunzi Belt are reported, from orthogneisses in the eastern part of the belt in northeastern Zimbabwe. Two intrusive masses of granitic and tonalitic gneiss, together with a granitic leucogneiss inferred to have formed by leucosome segregation during migmatisation, yield crystallisation ages of ca 2.62 Ga. These results are interpreted to date a major tectonothermal event along the northern margin of the Zimbabwe Craton, involving regional amphibolite-facies metamorphism and migmatisation, as well as emplacement of compositionally diverse granitoid plutons and retrogression of older granulites. This event is roughly coeval with orogenesis in the Limpopo Belt along the southern craton margin and with emplacement of the craton-wide ca 2.6 Ga Chilimanzi Granite Suite.